1. Field of the Invention
The present invention relates to error adjustment in direct conversion architectures. In particular, the invention relates to In-phase and Quadrature-phase based error detection and correction using an envelope based In-phase and Quadrature-phase extraction.
2. Description of the Prior Art
The use of digital wireless communication systems has recently been increasing. Systems of many different types have been introduced. For example, systems like Wireless LANs (Local Area Networks), digital radio DVB-T, UMTS and GSM are gaining more attention and users are given more alternatives in wireless communication. To get customers interested in new services it is essential that the equipment needed in order to use the services should be priced correctly. Transceivers with low cost and low power consumption are thus needed.
The Institute of Electrical and Electronics Engineers (IEEE) has developed a new specification 802.11a which represents the next generation of enterprize-class wireless local area networks (LANs). Among the advantages it has over current technologies are greater scalability, better interference immunity, and significantly higher speed, which simultaneously allows for higher bandwidth applications.
OFDM (Orthogonal Frequency Division Multiplex) is used as a new encoding scheme which offers benefits over spread spectrum in channel availability and data rate. Channel availability is significant because the more independent channels that are available, the more scalable the wireless network becomes. The high data rate is accomplished by combining many lower-speed subcarriers to create one high-speed channel. A large (wide) channel can transport more information per transmission than a small (narrow) one. The subcarriers are transmitted in parallel, meaning that they are sent and received simultaneously. The receiving device processes these individual signals, each one representing a fraction of the total data that, together, make up the actual signal. With many subcarriers comprising each channel, a tremendous amount of information can be sent at once.
The IEEE 802.11a wireless LAN standard defines a high system performance and therefore requires a certain signal accuracy for the OFDM transmitter output. Taking the analog base-band and radio frequency (RF) filter imperfections into account it is necessary to equalize the signal stream before transmission. The performance of a transmitter output signal is strongly dependent on the analog filter accuracy. To reach high signal accuracy, expensive and precise filters have to be used. However, in high volume products it is recommended to have those filters be as inexpensive as possible. It may be possible to insert low-cost and non-precise analog transmitter filters if a digital adaptive equalizer is installed to compensate for large amplitude ripple and group delay in the transmitter pass-band.
A solution in affordable transmitters is the use of a direct conversion analog front-end architecture in the transmitters. In the direct conversion solution, a digital base band signal is digital-to-analog converted and afterwards mixed into an RF signal. For the mixing process, two signals, a sine and a cosine signal, have to be provided. Because of technical reasons the precise orthogonality of both sinusoidal signals cannot be guaranteed; therefore an angle φ≠90° is measurable between the sine and cosine functions. This phenomenon is commonly called IQ phase imbalance. In addition, also an IQ amplitude imbalance arises between the I-branch and the Q-branch.
Moreover, analog base band components, such as analog filters, are always installed twice: one component for the I-branch and one component for the Q-branch. Because of manufacturing tolerances, different age or temperature influences, each component of a certain functional type may behave slightly differently compared with its counterpart on the other branch. Additionally, low-cost analog filters may contain amplitude ripple, non-linear phase and they may insert ISI (Inter Symbol Interference).
As an example, FIG. 1 shows a graph illustrating an I-branch and Q-branch ISI generated by analog filters in a direct conversion OFDM transmitter. No IQ phase or IQ amplitude imbalance errors are inserted so that only analog filter imperfections are visible.
The conjunction of frequency dependent base band devices with the constant IQ phase and amplitude imbalance imperfections results in frequency selective IQ phase and amplitude imbalance inaccuracies.
The phase and amplitude imbalance problem is present in any system employing direct conversion transmitters regardless of the modulation scheme or the multiple access solution. Particularly in a multicarrier system, such as WLAN, which uses OFDM, the problem is particularly severe, although it also affects single carrier systems, such as GSM or cable modems.
To provide the required high signal accuracy in transmitters in order to fulfill certain performance requirements at the receiver side it has to be guaranteed that analog direct conversion front-end imperfections, such as IQ phase and amplitude imbalance errors, will be minimal. So far, the solutions to the phase and amplitude imbalance problem have assumed the use of high quality analog base band components. Thus, the phase and amplitude imbalance correction methods have not taken frequency dependency into account. However, in low cost consumer appliances the use of high quality components is impossible. Therefore, the current correction methods do not present a solution to phase and amplitude imbalance correction in low cost receivers.
In addition, in direct conversion analog front end transmitters it is necessary to pre-correct the transmitted signal stream via fully digital adjustment loops. To find the appropriate error values the transmitter output signal has to be measured for example at the transmitter antenna input port and fed back to the transmitter digital domain.
The envelope of the high-frequency band signal can be measured by a detection diode. Envelope measurement has been done up to now via an amplitude level comparison. It has assumed that the envelope provides over a long period of time the same average value. This can be extracted from the measured analog signal and compared with the desired value. If the analog envelope average value is too low or too high certain adjustment algorithms can provide compensation.
With the invention of OFDM radios a much higher precision for the analog output signal is required. Hence algorithms for amplitude and phase imbalance adjustment have to become frequency selective to fulfill the requirements. But to run these algorithms there is much more envelope measurement precision required than to estimate whether the average amplitude is too high or too low. However, there is currently no other inexpensive, precise and stable solution to provide after that a complex base-band equivalent of the transmitted signal without installing a principle demodulator.